Fighting Ammonium Nitrate fires with water

What is it about?

Violent decomposition and explosion of ammonium nitrate (AN) induced by a fire present potentially a serious threat to personnel, facilities and nearby community. One of the most recent incidents involving AN occurred on April 17, 2013, in West, Texas, killing 15 people and injuring more than 250 people; this incident has caused heated discussion again on the safety issues associated with AN including firefighting issues. In terms of fire protection, water suppression systems have been widely used in chemical process facilities as an active protection layer, and they have been successful in tackling most of the fires. However, where water is the only agent to fight a fire in an AN store, acting as a cooling and hence combustion extinguishing agent, it does not limit the oxidant supply as this is contained within the AN molecule. Under some circumstances the addition of water may also favor the conditions for explosion. In this work the possible role of water interfering physically and to some extent chemically with AN stock in a fire is discussed, calling for more research to develop an optimal procedure to fight AN fertilizer fires.

Featured Image

Why is it important?

Ammonium nitrate (AN) is used extensively as a main component of mineral fertilizer, because it is an excellent source of nitrogen. Because of its wide use, it is present in large amounts stored in various central manufacturing and larger and smaller storage locations distributed over the country. AN is also widely used as an explosive base material usually mixed with a fuel, e.g., diesel, to be applied as ANFO in blasting work of various kinds, such as mining, quarrying, tunneling, and road construction. Pure AN, a salt of base ammonia, NH3, and nitric acid, HNO3, although not perceived as a dangerous material at atmospheric conditions is classified as a strong oxidizing agent in the Globally Harmonized System (GHS), the world’s classification system for hazardous materials. If in contact with a fuel AN is hazardous, because the AN molecule contains oxygen to reinforce combustion strongly. The problem with AN is that when heated, e.g., in a fire, at 170 °C it melts and decomposes into different gaseous products. If AN is pure the effect of heating is seen initially as small rising bubbles in the clear liquid and at escape giving off white fumes. When further heated in a sufficiently large quantity, confined by its own mass and by additional containment providing resistance to the flow of decomposition gas, its own constituents can react exothermally with each other further and the whole generates much more heat, brownish fumes, and in the end, produces fierce flames. When AN is contaminated with certain substances, such as chlorides, or with a combustible material it further amplifies the process. Over and over again it has been shown that pure AN and some of its mixtures when involved in a fire may detonate, although based on carefully designed tests these products are declared non-detonable or at least highly resistant to detonation. A relatively large number of incidents associated with AN have caused extensive loss of property and life. Attempts have been made to improve safety of AN based fertilizers by making non-detonable formulations. This can be achieved by diluting AN with inert materials with the disadvantage of lowering the fertilizing capacity. By manufacturing very hard non-porous prills even pure solid AN cannot be initiated to detonate by strong shock waves, at least not in kilogram-scale quantities confined in a steel tube up to 4 in. diameter. When heated in a fire, though, these prills will crack and lose their structure. In the wake of the West Fertilizer plant explosion on April 17, 2013, involving fertilizer grade, high density prills (FGAN), the safety issues related to AN hazards have again drawn attention including from the White House, which issued an Executive Order to review all existing regulations. Also, the Chemical Safety and Hazard Investigation Board (CSB) performed an extensive investigation. According to the mentioned GHS, and the linked UN Recommendations on the Transport of Dangerous Goods (UN TDG) with their tests and criteria, an AN containing product (contains NH4+ and NO3- ions) with 90% AN or more, and more than 0.2 % combustible/organic, is considered Class 1 Explosives. Division 1.1 of this class are the substances that have a mass explosion hazard, hence detonable, e.g., the high explosives. There is also Division 1.5 Very insensitive substances which have a mass explosion hazard. In here are the AN low density prills containing more than 0.2% C as total combustible/ organic used as blasting agents (if accepted by the prescribed test method). The very hard prilled AN products and in general ones with 90% AN or more, containing less than or equal to 0.2% C are considered non-detonable and are in Class 5.1, Oxidizing substances. Finally, AN compound fertilizers with less than 90% AN that are capable of self-sustaining decomposition, deflagration, also called cigar burning, producing toxic fumes are Class 9. Because AN has four crystal modifications, of which one is near ambient temperature (32 °C), due to cracks in the grains non-detonable formulations may become detonable after passing the modification temperature a number of times. Despite extensive research efforts the precise mechanism of how heated AN can transit from a rapid decomposition into a violent detonation is still unknown. Contamination can play a crucial role, but this may be limited to a catalyzing effect at the initial stages of decomposition. The following contaminants: chlorine, nitrocellulose, aromatic nitro compounds, etc., are typical in AN storage areas such as feeds stores. The element chlorine stems from chloride salts, such as potassium chloride known as potash. Prevention of AN incidents requires prevention of fires to initiate in or near an AN store. A fire free area requires the use of non-combustible construction materials and the avoidance of combustibles in contact with AN. Certainly for stores containing large amounts of AN, the use of sprinkler systems is recommended. Sprinkler systems to protect AN storage areas are suggested by agencies such as the Occupational Safety and Health Administration (OSHA) and the National Fire Protection Association (NFPA). NFPA 13 (standard for the installation of sprinkler systems) has been adopted as law by most local building code officials and fire marshals. As an active fire protection device, fire sprinkler systems have been successfully used to reduce fatalities and property losses. Worldwide, there are more than 40 million sprinklers fitted each year. Fire sprinkler systems have been effectively and efficiently operating during the past 120 years and improvements have been continuous up through today as new technology is developed. Sprinkler systems use water as the main extinguishing material. In most cases, the favorable physical properties of water make it an excellent material for fire suppression. Its heat capacity is relatively high, at 4.2 kJ kg-1 K-1. When evaporating, with the heat of vaporization of 2442 kJ kg-1, water will absorb large amounts of heat from the surroundings, such as flames and fuel. To be specific, water will expand 1700 times when it evaporates into vapor while diluting air oxygen and fuel vapors in the surrounding environment. In general, for fixed water suppression systems to work properly, the water supply should be adequate and reliable, and the water quantity should be adequate. Sprinkler systems are undoubtedly highly effective to control many types of combustion caused fires. However, it is still unclear whether the capability of sprinkler systems is sufficient to control AN-fires and prevent decomposition. In the work we discussed that water as an extinguishing agent for an AN-fire has limitations, in particular it can augment the fire when it is applied in insufficient quantities. For that, we considered AN’s explosion properties before the effects of water on decomposing AN. This work presents an overview of the main relevant findings already published, it added a few own thoughts especially with respect to the chemical and gas dynamic mechanism that might be active in triggering detonation in decomposing AN and to give suggestions for further research. All with the objective to obtain a clearer understanding of the effect of water on decomposing AN: Will water under expected circumstances increase the hazard or not?

Perspectives